In the posted irecv case if you are reading from the posted receive
buffer the problem is you may get one of three values:
1. pre irecv value
2. value received from the irecv in progress
3. possibly garbage if you are unlucky enough to access memory that is
at the same time being updated.
--td
Alberto Canestrelli wrote:
Thanks,
it was late in the night yesterday and i highlighted STORES but I
meanted to highlight LOADS! I know that
stores are not allowed when you are doing non blocking send-recv. But
I was impressed about LOADS case. I always do some loads of the data
between all my ISEND-IRECVs and my WAITs. Could you please confirm me
that OMPI can handle the LOAD case? And if it cannot handle it, which
could be the consequence? What could happen in the worst of the case
when there is a data race in reading a data?
thanks
alberto
Il 02/08/2010 9.32, Alberto Canestrelli ha scritto:
I believe it is definitely a no-no to STORE (write) into a send buffer
while a send is posted. I know there have been debate in the forum to
relax LOADS (reads) from a send buffer. I think OMPI can handle the
latter case (LOADS). On the posted receive side you open yourself up
for some race conditions and overwrites if you do STORES or LOADS from a
posted receive buffer.
--td
Alberto Canestrelli wrote:
Hi,
I have a problem with a fortran code that I have parallelized with
MPI. I state in advance that I read the whole ebook "Mit Press - Mpi -
The Complete Reference, Volume 1" and I took different MPI classes, so
I have a discrete MPI knowledge. I was able to solve by myself all the
errors I bumped into but now I am not able to find the bug of my code
that provides erroneous results. Without entering in the details of my
code, I think that the cause of the problem could be reletad to the
following aspect highlighted in the above ebook (in the follow I copy
and paste from the e-book):
A nonblocking post-send call indicates that the system may start
copying data
out of the send buffer. The sender must not access any part of the
send buffer
(neither for loads nor for STORES) after a nonblocking send operation
is posted until
the complete send returns.
A nonblocking post-receive indicates that the system may start writing
data into
the receive buffer. The receiver must not access any part of the
receive buffer after
a nonblocking receive operation is posted, until the complete-receive
returns.
Rationale. We prohibit read accesses to a send buffer while it is
being used, even
though the send operation is not supposed to alter the content of this
buffer. This
may seem more stringent than necessary, but the additional restriction
causes little
loss of functionality and allows better performance on some systems-
consider
the case where data transfer is done by a DMA engine that is not
cache-coherent
with the main processor.End of rationale.
I use plenty of nonblocking post-send in my code. Is it really true
that the sender must not access any part of the send buffer not even
for STORES? Or was it a MPI 1.0 issue?
Thanks.
alberto
_______________________________________________
users mailing list
users_at_[hidden]
http://www.open-mpi.org/mailman/listinfo.cgi/users
--
Oracle
Terry D. Dontje | Principal Software Engineer
Developer Tools Engineering | +1.650.633.7054
Oracle * - Performance Technologies*
95 Network Drive, Burlington, MA 01803
Email terry.don...@oracle.com <mailto:terry.don...@oracle.com>
